Sticktion is a phenomenon which occurs in a slow-moving mechanical apparatus. Sticktion often occurs during the control of torque devices to position heavy mechanisms. It is a sharp rise in coulomb friction due to micro-welds formed between two sliding surfaces. This has been modeled as a step in coulomb friction from zero velocity to some small "breakaway" velocity value, and the complement in the opposite direction. This creates a constant force resisting the motion until a minimum "breakaway" velocity is reached.
Conventional linear controllers can be optimized for a given inertial load and viscous frictional drag. Since they are tuned to a specific set of parameters, these controllers react sub-optimally when the actual system parameters change. The controller's ability to perform under these changing parameters is characterized as its "robustness".
A robot is a special type of controlled torque device which is often utilized to position heavy mechanisms. The number of joints of commercially available robots varies from three to seven. Typically they have six joints, giving six degrees of freedom, with a gripper which is referred to as a hand or an end effector. Each joint of the robot is positionally controlled with a feedback loop. Typically the device control is done entirely at the joint level. High-level path control of position and orientation of the hand is done in Cartesian coordinates.
Most robot controllers use a proportional-derivative type of control method. Such a control is generally termed a PD control. A PD control keeps each robot axis in control by taking position error information multiplied by a constant and adding the resulting amount to the velocity multiplied by another constant. The major advantages of this system is its stability over a wide range of operating conditions. The disadvantages of such a system are a large amount of following error and steady state error.
Typically, such errors affect robot coordinated motions and repeatability up to one millimeter or more. In many applications in robotics, such a large repeatability figure is intolerable and expensive tooling modifications are needed to compensate for it.
Other techniques used by some manufacturers and research laboratories include the use of an integrator to compensate for the shortcoming of the PD controls. The resulting control is called a proportional-integrator-derivative or PID control. PID controls have the advantage of eliminating steady state error. However, PID controls have the disadvantages of difficulty in tuning the parameters, overshoot and oscillations.
The U.S. Patent to Kubo et al U.S. Pat. No. 3,781,626 disclosed an optimized PID controller. Control coefficients are generated by a computing network which are used to modify the operation of the central control unit is response to the changing characteristics of the remote control device.
The U.S. Patent to Kurakake discloses a position control system having a closed loop in which an integrating element and a device for compensating for unstableness of the closed loop caused by the integrating element are provided.
The U.S. Patent to Inaba et al U.S. Pat. No. 4,374,349 discloses a control circuit including an error register. When the error value within the error register becomes higher than a predetermined value, the multiplication factor of the position gain multiplier is increased.
The U.S. Patent to Pollard et al U.S. Pat. No. 4,362,978 discloses a control system utilizing a variable inertia scaling factor accomplished through the use of a look-up table with appropriate interpolation table entries.
The U.S. Patent to Dunne U.S. Pat. No. 4,510,428 discloses a control system for a hydraulic actuator wherein variable inertia scaling of selected loop command signals is provided.
Other United States patents disclosing various control circuits include the U.S. Patents to Kade et al U.S. Pat. No. 4,540,923, Salemka U.S. Pat. No. 4,498,036, Chitayat et al U.S. Pat. No. 4,494,060, Cook et al U.S. Pat. No. 4,491,718 Crimshaw U.S. Pat. No. 4,479,176, Bennett et al U.S. Pat. No. 4,463,297, Whitney et al U.S. Pat. No. 4,458,321, Takemoto U.S. Pat. No. 4,507,594, Mitsuoka U.S. Pat. No. 4,437,045, Kolell et al U.S. Pat. No. 4,041,287 and Engelberger et al U.S. Pat. No. 4,132,937.
It is desirable that a controlled torque device have the following steady state performance criteria (1) a zero error; (2) fast rise and settling times, (3) little to no overshoot, and (4) a robust reaction to disturbance and sticktion. The accomplishment of criteria (1) and (4) is often accomplished by providing a high gain linear integrator in the subject control system. However, this results in losing the second and third performance criteria which are high-speed dynamic response characteristics. Criteria (1) and (4) are typically low-speed, steady state criteria.